Electronic devices are becoming thinner with bigger screens. So Chip On Flex (COF) with fine lines and good flexible is widely used in the electronic device. COF is difficult to connect with Surface Mount Technology (SMT). The COF can be combined with at least one flexible circuit board, and when the COF is combined with a flexible circuit board, the COF is manufactured to have a size correspondingly to the flexible board. However, the COF has only some of the fine conductive lines coupled with the flexible circuit board. The COF in this usage thus has a lower use ratio when combined with the flexible circuit.